The present invention relates generally to drilling subterranean boreholes and, more specifically, to a downhole motor for drilling curved and horizontal boreholes.
Subterranean boreholes are typically drilled using a drill bit connected to a distal end of a drill string made up of coiled tubing or sections of drill pipe connected in series by drill collars. The drill bit is rotated by rotating the entire drill string, and/or the drill bit is rotated by a downhole motor which is included as a component of the drill string. Drilling fluid is typically pumped down through the drill string to the bottom of the borehole and back up the annulus between the drill string and the wall of the borehole. The drilling fluid cools the drill bit and removes the cuttings resulting from the drilling operation. In the instances where the drill bit is rotated by a downhole motor, such as a progressing cavity motor, the drilling fluid can also supply hydraulic power to operate the downhole motor.
In addition to rotating the drill bit to progress the borehole and the drill string into a formation, the downhole motor can be configured to control and orientate the drilling direction of the drill bit, such as in the case of a bent downhole motor. Bent downhole motors generally include a bend that allows the bent downhole motor to guide the drill bit and drill string along a curved path away from a vertical drilling direction to a horizontal drilling direction. The curved path is generally considered to have a short radius curve when the size of the radius of the curved path is less than approximately 80 feet (24 meters). The curved path is generally considered to have a long radius curve when the size of the radius of the curved path is greater than approximately 800 feet (240 meters). The curved path is generally considered to have a medium radius curve when the size of the radius is anywhere between that of a short radius and a long radius. Boreholes with medium radius curves and short radius curves can be technically more challenging and costly to drill than a borehole with a long radius curve, however, a borehole with a long radius curve can be impractical or impossible to drill where property and lease boundary restrictions limit the horizontal space in which the borehole can extend to reach a target.
Typically, drilling operators limit the use of short radius and medium radius curves to boreholes with diameters smaller than 9 inches (23 centimeters), such as a borehole with a diameter of 8.25 inches (20.9 centimeters). While drilling boreholes with a short radius or medium radius curve and a diameter larger than 9 inches (23 centimeters) are easier to case (due to less friction and drag between the case and the wall of the borehole) and allow a larger variety of case sizes than a borehole with a similar curve but a smaller borehole diameter, drilling operators do not typically drill boreholes with a short radius or medium radius curve and a diameter larger than 9 inches (23 centimeters) because drilling a borehole with a diameter larger than 9 inches (23 centimeters) and with a short or medium radius curve is more time consuming and therefore more costly.